The present invention is related to a method for non-contact measurement of the velocity and/or the length of an extrudate moved forward in the longitudinal direction, in particular of a cable.
There is a high demand to measure the length of produced extrudates accurately, like for instance of wires, cables, pipes or tubes. For instance, a sheathing insulation plant produces cables having a value of about 30 million ε per year. Every saving with respect to the produced amount means an additional added value. For instance, 30 000,00 ε can be saved if the produced length can be reduced about 0.1%.
It is known to perform a non-contact length measurement on extrudates or webs after the principle of frequency shift (Doppler Effect). In the measurement of so-called web material, like paper, steel plate, fabrics or the like, this method has outstandingly proven its worth. However, there are problems with length measurements on extrudate-shaped products like cables for instance.
Therefore, the present invention is based on the objective to provide a method for non-contact measurement of the velocity and/or the length of an extrudate moved forward in the longitudinal direction, in particular of a cable, by which accurate measurement results can be obtained that are as free of errors as possible.
The present invention starts from the finding that a diffraction pattern which is generated by a limited length of the edge of the extrudate is characteristic for this length portion of the extrudate and differs from diffraction patterns that are caused by subsequent length portions of the extrudate. When it is detected, with the aid of the receiving surfaces of a suitable image sensor and by analysing the diffraction pattern, that an once acquired diffraction pattern or a characteristic trait of the diffraction pattern, respectively, appears again at a spatially remote location, the velocity in which the extrudate has covered the path between the first and the second location can be determined from the time that passes between the first acquisition of the diffraction pattern and its coincidence with a diffraction pattern acquired at the second location, and the distance of the locations. By continuously repeating the described process, the extrudate velocity can be determined at every point in time, and an accurate length determination can be performed by way of this, even when velocity fluctuations occur during the advancing.
The utilisation of diffraction patterns for purposes of measurement has become known from EP 0 924 493. In the known measurement method, the diameter of an extrudate, of a cable with small diameter in particular, is determined by directing a fan-shaped beam of a monochromatic, punctiform light source to the extrudate, transversely to the same. The shadow of the extrudate is projected onto a row-shaped light sensitive sensor of per se known construction. The measured signals of the sensor, caused by diffraction, yield information about the position of the diffraction edge in the measurement space. Due to diffraction phenomena, geometric shadow boundaries do not result directly from the intensity distribution of the radiation hitting the sensor indeed; however, they can be derived from the resulting diffraction fringes. In the invention, the diffraction patterns, i.e. characteristic traits of the measured intensity distributions, are used for marking length units of the surface of an extrudate, in order to determine in which period of time the marked surface moves from a first to a second location. The arrival on the second location is detected by a sufficient coincidence of characteristic traits of a diffraction pattern acquired at the first location with characteristic traits of a diffraction pattern acquired at the second location. A correlation between first and second diffraction patterns or their characteristic traits, respectively, is preferably determined with the aid of the so-called cross correlation function. In signal analysis, the cross correlation function is used for describing the correlation between two signals at different time shifts between the signals, as is well known.